"Intracellular crystals of calcium salts, such as calcium oxalate, are fairly
common throughout the plant kingdom. Under a compound microscope, the glistening
crystals resemble many-faceted diamonds. The stems of some bamboos, including
spiny bamboo (Bambusa bambos), contain silica concretions composed of silicon
dioxide. Some palm seeds contain vegetable ivory, hardened endosperm tissue
containing a polysaccharide called hemicellulose. Like wood, vegetable ivory
is essentially composed of dead cells; however, unlike grainy hardwoods it
has a texture and hardness similar to ivory. In fact, vegetable ivory is
remarkably dense, with a rating of roughly 2.5 on the scale of mineral hardness.
Ivory-nuts can be polished in a stone tumbler, as you would polish agates
and quartz, or by using tin oxide and a buffing wheel. Perhaps some reports
of hard, white objects inside coconuts are remnants of dried endosperm tissue."